1. Field of the Invention
This invention relates to a fuel hose of low fuel permeability. More particularly, it relates to a hose which can maintain an excellently low level of fuel permeability even when it is subjected to a bending stress in practical use. The term xe2x80x9cfuelxe2x80x9d as herein used includes methanol, hydrogen gas, and other fuel for a fuel cell.
2. Description of the Related Art
Stricter restrictions have recently come to be imposed upon the leakage of fuel from motor vehicles, and brought about a strong demand for hoses of low fuel permeability, since fuel hoses are highly responsible for the leakage of fuel from motor vehicles. As a result, there have been made a large number of proposals covering fuel hoses having at least their innermost wall layers formed from a resin of low fuel permeability. Every such hose usually has a multilayer wall formed solely from resin layers, or including an outermost layer formed from a rubber material.
The fuel hoses are usually classified into liquid-line and vapor-line hoses, depending on the fluid which each hose is used to convey. The inside diameter of each hose, its shape, etc. are decided depending on the kind of fuel to be conveyed, its flow rate and the ease of hose assembly. The shape of the hose may be, for example, straight or bent, or smooth or corrugated.
The resin forming the innermost wall layer of a fuel hose is preferably a fluororesin, such as an ethylene-tetrafluoroethylene copolymer (ETFE), a tetrafluoroethylene-hexafluoropropylene-vinylidene fluoride terpolymer (THV), polyphenylene sulfide (PPS), polybutylene naphthalate (PBN) or polyvinylidene fluoride (PVDF), though specific kinds of other resins may be equally useful. Hoses of low fuel permeability having wall layers formed from fluororesins are disclosed in, for example, U.S. Pat. Nos. 5,884,671, 5,884,672, WO 98/5557, WO 99/45044, JP-A (Japanese Patent Application Laid-Open No.)-10-311461, JP-A (KOHYO)-10-503236, U.S. Pat. No. 5,566,720 and JP-A-7-173446. Hoses of low fuel permeability having wall layers formed from PPS are disclosed in, for example, JP-A-10-138372, JP-A-10-230556, JP-A-10-296889 and JP-A-2000-63669. Hoses of low fuel permeability having wall layers formed from PBN are disclosed in, for example, JP-A-2000-154890.
Fuel hoses are usually laid in a bent state in a motor vehicle for reasons of layout, or to allow displacement resulting from the collision of the vehicle. The inventors of this invention, have, however, found experimentally that a bent hose has craze in its wall, though it may not have any visible cracks. Craze is injurious to the low fuel permeability of the hose. None of the prior proposals, however, suggest any specific or effective means for overcoming such a defect of a bent hose.
It is, therefore, an object of this invention to provide a hose of low fuel permeability having specific and effective means for overcoming any adverse effect exerted by a bending stress upon its low fuel permeability.
According to a first aspect of this invention, there is provided a hose of low fuel permeability having a wall including an inner layer of a resin, wherein the hose has a fuel permeability in a bent state with R50 not more than three times a fuel permeability in its straight state. The bent state with R50 means that the hose has its longitudinal axis bent in a manner to form an arc having a diameter of 100 mm. The inventors experiments have revealed that the hose as defined above can maintain a practically satisfactorily low fuel permeability when it is subjected to various bending stresses in actual use. Thus, the hose of the first aspect is expected to have excellent low fuel permeability owing to its resin inner layer, and, due to its low fuel permeability in its bent state, the deterioration of low fuel permeability caused by bending is suppressed within a practically allowable range.
According to a second aspect of this invention, the hose is permeable to not more than 0.3 mg of fuel per square centimeter of its inner wall surface per day, as tested under specific conditions in its bent state with R50. Referring to the xe2x80x9cspecific conditionsxe2x80x9d, the hose is filled with FC/E10 (which will be explained in the description of examples), and left to stand at a temperature of 40xc2x0 C. for 168 hours. Then, the hose is emptied, filled with a fresh supply of FC/E10, weighed, and left to stand at 40xc2x0 C. for 72 hours. Then, it is weighed again, its weight is compared with its prior weight and their difference is used to calculate the weight (mg) of the fuel lost per square centimeter of its inner wall surface per day. The practically allowable fuel permeability of any hose depends on the purpose for which it is used, but the hose according to the second aspect of this invention is satisfactory for a wide range of practical uses as a fuel hose.
According to a third aspect of this invention, the resin forming the inner wall layer of the hose is ETFE, THV, PPS, PBN or PVDF. Owing to its high fuel impermeability, every such resin is suited to realize the low fuel permeability of the hose in its bent state as in the first or second aspect.
According to a fourth aspect of this invention, the resin of the inner layer is so high in electrical conductivity as to have a volume specific resistance not exceeding 109 ohms cm. The hose is not electrically charged to produce any spark when it is used to supply fuel to a gasoline tank, or when the engine is driven. The hose is, however, likely to form craze when bent, and it is, therefore, recommended that its features be combined with those of the hose according to the first or second aspect of this invention.
According to a fifth aspect of this invention, the hose has a corrugated wall along at least a part of its length. The corrugated wall is so adaptable to a bending force as to ensure the low fuel permeability of the hose to a further extent when it is bent.
According to a sixth aspect of this invention, the resin forming the inner wall layer of the hose has a melt viscosity as listed below at a shear rate of 100 Sxe2x88x921:
ETFE: 500 to 3000 Paxc2x7s (300xc2x0 C.)
THV: 500 to 3000 Paxc2x7s (280xc2x0 C.)
PPS: 500 to 3000 Paxc2x7s (310xc2x0 C.)
PBN: 500 to 3000 Paxc2x7s (270xc2x0 C.)
PVDF: 1000 to 3000 Paxc2x7s (210xc2x0 C.)
The adjustment of the melt viscosity of the resin as listed above is one of particularly preferable examples of specific means for realizing the low fuel permeability of the hose in its bent state. If the resin has a higher viscosity, a low fuel permeability of the hose is less deteriorated when it is bent, but any resin having an extremely high melt viscosity is difficult to extrude.
According to a seventh aspect of this invention, the low fuel permeability of the hose in its bent state or the adjustment of the melt viscosity of the resin is made by:
(1) adjusting the degree of polymerization of the resin; or
(2) grafting or copolymerizing a relatively long side chain to the main chain in each molecule of the resin, or crosslinking the resin. In either event, an increase in the mutual entanglement of macromolecules in the resin brings about a change of its melt viscosity to enable the formation of a resin layer which hardly forms any craze when the hose is subjected to a bending stress, thereby ensuring its low fuel permeability in its bent state.
According to an eighth aspect of this invention, the resin has a volume swelling degree as listed below after 360 hours of immersion in Fuel C (a mixture containing toluene and isooctane in a ratio of 50:50 and further containing 10% by volume of ethanol) at 60xc2x0 C.:
ETFE: 3.0% or less;
THV: 3.0% or less;
PPS: 20.0% or less;
PBN: 7.0% or less;
PVDF: 7.0% or less.
The adjustment of the volume swelling degree of the resin as listed above is another preferable example of specific means for ensuring the low fuel permeability of the hose in its bent state.
According to a ninth aspect of this invention, the low fuel permeability of the hose in its bent state or the adjustment of the volume swelling degree of the resin is made by:
(3) copolymerizing a monomer having fuel resistance in the resin; or
(4) blending a polymer having fuel resistance in the resin. In either event, a reduction in the compatibility of the resin with fuel brings about a reduction of its volume swelling degree to thereby make the hose hardly permeable to any fuel when it is subjected to a bending stress.
According to a tenth aspect of this invention, the low fuel permeability of the hose in its bent state is realized by:
(5) surrounding the inner layer by an outer layer of a resin having a flexural modulus of 600 MPa or below;
(6) reducing the residual oligomer and monomer in the resin forming the inner layer to a total of 2.0% by weight or less; or
(7) adjusting the crystallinity of the resin forming the inner layer to 25% or above.
These are other preferable examples of specific means for ensuring the low fuel permeability of the hose in its bent state. The outer layer is flexible enough to adapt the hose satisfactorily to a load of bending stress and enable it to maintain its low fuel permeability when it is bent. The outer layer is also very effective for absorbing a bending stress to reduce any stress acting upon the inner layer. As the residual oligomer and monomer are considered to be one of the causes for any craze, their reduction makes craze less likely to occur and thereby enables the hose to maintain its low fuel permeability when it is bent. The increase of the crystallinity of the resin brings about a reduction of the amorphous phase in which craze is likely to occur under a bending stress, so that the hose may be able to maintain its low fuel permeability when it is bent.
According to an eleventh aspect of this invention, the hose is made by employing at least two of the seven means (1) to (7) recited in the seventh, ninth and tenth aspects of this invention. All of the seven means are effective even if they are used independently of one another. The means (1) to (4) are particularly effective. It is, however, still more effective to use at least two of the seven means (1) to (7) simultaneously.
According to a twelfth aspect of this invention, the hose has an outermost wall layer formed from a rubber material.
The above and other features and advantages of this invention will become more apparent from the following description.